Measuring the topography of a sample is important for example in the manufacture of semiconductor devices, where a semiconductor wafer typically undergoes Chemical Mechanical Planarization (CMP) after a photolithography-etching-deposition process CMP might result in the wafer's topography induced by erosion of densely packed small feature arrays and dishing of larger features, while the ideal CMP results would be a substantially flat surface of the wafer.
Techniques enabling the topography measurements have been developed and are disclosed for example in U.S. Pat. Nos. 5,307,152, 5,671,050, 5,969,819, and 6,084,712. All these techniques utilize two diffraction gratings, and according to all these techniques, except for that of U.S. Pat. No. 5,671,050, two gratings are located, respectively, in spatially separated optical paths of illuminating and reflected light. According to the technique of U.S. Pat. No. 5,671,050, two gratings are located in the path of the incident and returned light. This is implemented locating both gratings very close to the sample.
The use of a single grating has been proposed in the following articles W. Jaerisch and G. Makosch, “Optical contour mapping of surfaces” (Applied Optics 12(7), 1552-1557 (1973), and P. Jacquot, X. Colonna de Lega and P M Boone. “Common-path interferometer for flatness testing” SPIE 2248, Optics for productivity in manufacturing, paper 18 (1994). Here, similarly to the technique of U.S. Pat. No. 5,671,050, the grating is located close to the sample (downstream of all other elements of the optical system).